∴∠BAE=∠DAF，

∫ square ABCD,

∴∠ABE=∠D=90，AB=AD，

∴△ABE≌△ADF，

∴S quadrilateral AECF=S quadrilateral AFCB+S△ABE=S quadrilateral AFCB+S△ADF=S square ABCD,

The side length of a square is 3,

∴S quadrilateral AECF=S square ABCD = 9.

So the answer is 9.